1. Field of the Invention
The present invention relates to X-ray diagnostic apparatus which can correct a signal output by the X-ray detector.
2. Description of the Related Art
Heretofore, an X-ray photofluorographic imaging apparatus provided with an image intensifier (I.I.)-TV camera system has been used as X-ray imaging means. The I.I., as its schematic sectional view is shown in FIG. 1, comprises: an input fluorescent plane that converts an incident X-ray into an image of visible radiation; a photoelectric conversion film that converts the light intensity distribution of the visible radiation image and to which cathode potential is given; an anode to which an accelerated field accelerating electron beams irradiated from the photoelectric conversion film is given; focusing electrode that focuses the electron beams on the output fluorescent plane; and the output fluorescent plane to which the accelerated electron beams are made incident and converted into an optical image again. Then, the optical image formed on the output fluorescent plane is amplified to a luminance a few thousand times as the optical image on the input fluorescent plane. The image whose luminance has been amplified is projected on a monitor unit through the TV camera or stored in an image storage unit.
In recent years, on the other hand, an X-ray detector in which a semiconductor is used in an X-ray detection part has been proposed from a viewpoint of manufacturing a small and thin detector.
As the configuration of the X-ray detector using the semiconductor, an indirect conversion type X-ray detector (U.S. Pat. No. 4,689,487) and a direct conversion type X-ray detector (U.S. Pat. No. 5,319,206) and the like are proposed.
The indirect conversion type X-ray detector converts the X-ray into light via a chemical such as cesium iodide (CsI) crystal, converts the light intensity into electric charge by the photoelectric conversion operation of a photodiode, and stores the electric charge in a capacitance for every pixel. Then, switching means such as a thin film transistor (hereinafter, abbreviated as TFT) matrix sequentially reads out the accumulated electric charge, a charge amplifier (also called an initial step integrating amplifier) converts it into a voltage, and the voltage is subject to an A/D conversion to obtain a digital image signal.
On the other hand, as shown in the schematic sectional view of FIG. 2A, in the direct conversion type X-ray detector, the X-ray made incident to the semiconductor such as selenium (Se) under high field contributes to occurring of the electric charge due to a direct photoelectric effect, and thus the electric charge is accumulated in a signal accumulation capacitance for every pixel. Then, similarly to the direct conversion type X-ray detector, the accumulated electric charge is sequentially read out by the switching of the TFT, converted into the voltage by the charge amplifier (not shown), and the voltage is subject to the A/D conversion to obtain the digital image signal.
Furthermore, when diagnosis is performed using the foregoing X-ray detector, the sensitivity of a plurality of detection devices provided in the X-ray detector has dispersion. Therefore, a sensitivity correction table 103 and an offset correction table 102 have been conventionally used as shown in a block diagram of FIG. 2B in order to correct the sensitivity.
Namely, the sensitivity correction table 103 is the one where a sensitivity characteristic in the X-ray detector is measured in advance and different sensitivity correction coefficient for every pixel (for every detected device) is stored. The sensitivity correction table 103 outputs a detection value which is corrected by multiplying the coefficient by the output of the X-ray detector. In addition, the offset correction table 102 is the one where an offset characteristic in the X-ray detector is measured in advance and different offset correction coefficient for every pixel is stored. The offset correction table 102 outputs a detection value which is corrected by subtracting the coefficient from the output of the X-ray detector 101.
However, the inventors found out a phenomenon that the sensitivity temporarily reduces and the offset increases in accordance with the intensity of the X-ray when the X-ray having intensity of a predetermined value or more is made incident, in the X-ray detector, among others, the X-ray detector using the semiconductor. In other words, this means that the sensitivity or the offset changes with time in each pixel.
This phenomenon is conspicuous when the conditions of the X-ray intensity, that is, an X-ray quantity being a product between an X-ray tube current and exposure time, are greatly different. For example, in a fluorography where relatively light X-ray is continuously irradiated, the phenomenon appears remarkably when a time of X-ray exposure is relatively long, and in an imaging where relatively strong X-ray is intermittently irradiated, the phenomenon appears remarkably when strong X-ray exposure is irradiated. It is to be noted that there is a case where a few hundred times of difference of a maximum electric charge accumulated in one pixel between the fluorography and the imaging.
The decrease of this temporary sensibility and the increase of the offset causes superimposing the after-image which is based on the X rays detected the last time and called ghost on the regular image acquired by fluorography or imaging.
Therefore, the object of the present invention is to solve the foregoing problems, and to provide an X-ray diagnostic apparatus and a control method thereof configured to acquire an image by properly correcting the output of the X-ray detector even when the temporary reduction of the sensitivity (hereinafter, may be referred as the sensitivity) or increase of the offset occurs.
According to an aspect of the present invention there is provided an X-ray diagnostic apparatus comprises: a detector configured to detect an incident X-ray; an operator configured to estimate a characteristic change of the X-ray detector based on a value of a signal previously detected the detector at least last time; and a first correction device configured to execute a correction by which the estimated characteristic change is cancelled to a signal output from the X-ray detector.
According to the second aspect of the present invention there is provided an X-ray diagnostic apparatus comprises: a X-ray detector having a plurality of semiconductor elements arranged in the shape of a 2-dimensional matrix and configured to detect an incident X-ray and generate electric information; an operator configured to estimate a characteristic change of each of the semiconductor elements based on a value of a signal previously detected by each of the plurality of the semiconductor elements at least last time; and a correction device configured to execute a correction by which each of the estimated characteristic change is cancelled to each of the signals output from each of the plurality of the semiconductor elements.
According to the third aspect of the present invention there is provided an X-ray diagnostic apparatus comprises: a X-ray detector having a plurality of semiconductor elements arranged in the shape of a 2-dimensional matrix and configured to detect an incident X-ray and generate electric information; a memory configured to store a correlation relation between an offset characteristic and a sensitivity characteristic of each of the semiconductor elements; a first operator configured to estimate an offset characteristic change of each of the semiconductor elements based on a value of a signal previously detected by each of the plurality of the semiconductor elements at least last time; a second operator configured to estimate a sensitivity characteristic change of each of the semiconductor elements from the estimated offset characteristic change according to the correlation relation; and a correction device configured to execute a correction by which each of the sensitivity characteristic change is cancelled to each of the signals output from each of the plurality of the semiconductor elements.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.